A petroleum (or oil and gas) reservoir is a subsurface pool of hydrocarbons trapped in subsurface porous rock formations. Oil and gas wells are often drilled into these subsurface reservoirs to extract the trapped hydrocarbons. It can be beneficial to understand the characteristics of rocks penetrated by the well, including the characteristics of the formation surrounding the well, as knowledge of the characteristics can help with critical decisions that need to be made during completion and production of the well. For example, reservoir characteristics can be used to determine whether the formation contains hydrocarbons, to estimate the amount of hydrocarbons in the formation, to predict the ability to extract (or produce) the hydrocarbons, and to determine optimal techniques for drilling the well and producing the hydrocarbons from the well.
Reservoir characteristics of interest can include formation porosity, formation permeability, resistivity, water saturation, free water level (FWL), and the like. Porosity indicates how much space exists in a particular formation, where oil, gas, and/or water may be trapped. Permeability indicates the ability of liquids and gases to flow through the formation. Resistivity indicates how strongly the formation (rock and fluids) opposes the flow of electrical current, and can be indicative of the porosity of the formation and the presence of hydrocarbons. For example, resistivity may be relatively low for a formation that has high porosity and a large amount of water, and resistivity may be relatively high for a formation that has low porosity or contains a large amount of hydrocarbons. Water saturation indicates the fraction of water in a given pore space. FWL is a level (or depth) below the lower boundary of the hydrocarbons in the reservoir, and at which the capillary pressure between water and oil is zero. Above the FWL, the reservoir is expected to produce hydrocarbons or water depending on oil and water relative permeability; below the FWL, the reservoir can produce only water. Reservoir characteristics can be determined using a variety of different techniques. For example, certain characteristics can be determined via coring (e.g., physical extraction of rock samples) or logging operations (e.g., wireline logging, logging-while-drilling (LWD) and measurement-while-drilling). Coring operations include physically extracting a rock sample from the target reservoir through a wellbore for detailed laboratory analysis. For example, when drilling an oil or gas well a coring bit can cut plugs (or “cores”) from the formation and bring them to the surface, and these samples can be analyzed at the surface (e.g., in a lab) to determine various characteristics of the formation at the location where the sample was taken from. Although a coring approach can be very effective in determining reservoir characteristics, it can be time consuming and expensive. Logging operations typically include lowering one or more measurement tools into a wellbore, and recording measurements as the tool traverses the wellbore. The plot of the measurements versus depth is referred to as a “log”. Logs can be analyzed to determine some characteristics of the well (e.g., including characteristics of the reservoir penetrated by the well), while other characteristics may be difficult to determine using only logs.
There are many different types of logging available, and a particular form of logging may be selected and used based on the logging conditions and the type of measurements to be acquired. For example, nuclear magnetic resonance (NMR) logging measures the induced magnetic moment of hydrogen nuclei (protons) contained within the fluid-filled pore space of porous media (reservoir rocks). Unlike some conventional logging measurements (e.g., acoustic, density, neutron, and resistivity), which respond to both the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR logging measurements respond to the presence of hydrogen protons only. Because these protons primarily occur in pore fluids, NMR effectively responds to the volume, composition, viscosity, and distribution of these fluids (e.g., oil, gas, water). NMR logs provide information about the quantities of fluids present, the properties of these fluids, and the sizes of the pores containing these fluids, and using this information, it may be possible to infer or estimate the volume (porosity) and distribution (permeability) of the rock pore space, and the like. With regard to measurement of the nuclear magnetic properties of formation hydrogen, the basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus decay time at each sample depth, typically from about 0.3 ms (milliseconds) to about 3 s (seconds). The T2 decay is processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. As a further example of logging techniques, resistivity logging measures the electrical resistivity of rock or sediment in and around a borehole. Resistivity measurements obtained via such logging can be used to determine corresponding reservoir water saturation (Sw). Accordingly, resistivity logging can be used to generate corresponding water saturation (Sw) logs along a wellbore.